Total hip replacement surgery is commonly performed to alleviate pain and loss of function in injured or diseased hip joints. During this surgery, the articulating surfaces of the hip joint are replaced with prosthetic bearing components. The replacement components generally include a femoral component having a convex bearing surface and an acetabular cup component having a mating concave bearing surface.
Modular prosthetic components have become popular because they allow the surgeon to assemble components in a variety of configurations at the time of surgery to meet specific patient needs and surgeon preferences. For example, modular acetabular components generally include separate shell and liner components that can be assembled in a variety of configurations of shell surface finish, shell outer diameter, liner inner diameter, and liner bearing material. With a modular acetabular component, it is desirable to lock the shell and liner together to prevent expulsion of the liner and to minimize debris producing wear between them. Typically, the engagement mechanism is formed adjacent the equator of a hemispherical shell and liner to maximize the engagement area and the resulting holding power of the engagement mechanism.
Various liner bearing materials are in use. The liners vary in hardness, friction coefficient with different paired ball heads, weight, and wear resistance. Polymers, including ultrahigh molecular weight polyethylene (UHMWPE), are commonly used as bearing materials paired with an opposing metal, ceramic, or other composition ball head. The wear resistance of UHMWPE has been improved by irradiating it to cause changes in its chemical and mechanical properties. As the wear properties are improved the bulk physical properties change also. Other materials, including metals and ceramics, have also been used for acetabular bearings. These materials vary from one another in terms of their hardness, resilience, brittleness, and other physical properties. Because of this variation, various mechanisms have been developed for engaging acetabular liners with their mating shells. Different engagement mechanisms are suitable for different liner and shell material combinations. These engagement mechanisms include snap-fit, cylindrical press-fit, taper-fit, threaded engagement, and other suitable locking mechanisms. It is desirable to be able to alternately fit different liners into a common shell to reduce inventory while allowing surgeon choice in liner selection. It is also desirable to allow intraoperatively changing from one liner to another without having to remove a shell that has already been placed in the surgical site during a primary surgery or one that has become well fixed and only needs liner replacement in a revision surgery.
U.S. Pat. No. 6,475,243 issued to Sheldon et al. Nov. 5, 2002. The '243 patent teaches a shell and liner arrangement that permits alternative engagement of liners made of different materials and having different engagement mechanisms. The '243 shell includes both a snap-fit engagement mechanism and a taper-fit engagement mechanism formed in the interior of the shell adjacent the shell equator. The snap-fit mechanism includes a pair of annular grooves formed in the shell for receiving a pair of annular projections protruding from the liner. One of the annular projections deforms upon insertion of the liner into the shell and snaps back into engagement with one of the annular grooves to retain the liner in the shell. The other annular projection seats in the other annular groove and engages a projection formed in the groove to prevent rotation of the liner relative to the shell. The snap-fit mechanism is suitable for relatively soft liner materials that can deform to snap into the annular ring and that can deform to engage the antirotation feature.
The taper-fit engagement mechanism includes a tapered seat formed on the inside of the shell adjacent the equator for receiving a liner having a tapered exterior surface. The taper-fit engagement mechanism is suitable for relatively hard liner materials. The annular grooves of the snap-fit engagement mechanism are superimposed with the taper-fit engagement mechanism such that the annular grooves interrupt the tapered seat and consequently reduce the bearing area of the tapered seat and potentially create stress risers at the taper surface.
U.S. Pat. No. 6,610,097 issued to Serbousek et al. Aug. 26, 2003. The '097 patent teaches a conventional hip cup arrangement including a metal shell and a polyethylene bearing insertable into the shell. The polyethylene bearing is part of a subassembly including a metal liner permanently attached during manufacture to the polyethylene bearing. The liner provides a metal taper surface to facilitate taper seating of the subassembly into the shell.